Direct Numerical Simulations of Transitional Separation Bubble Development in Swept Blade Flow Conditions

Abstract

This paper describes numerical simulations of the instability mechanisms in a separation bubble subjected to a threedimensional freestream pressure distribution. Two direct numerical simulations are performed of a separation bubble with laminar separation and turbulent reattachment under low freestream turbulence at flow Reynolds numbers and streamwise pressure distributions that approximate the conditions encountered on the suction side of typical lowpressure gasturbine blades with blade sweep angles of 0 deg and 45 deg. The threedimensional (3D) pressure field in the swept configuration produces a crossflowvelocity component in the laminar boundary layer upstream of the separation point that is unstable to a crossflow instability mode. The simulation results show that crossflow instability does not play a role in the development of the boundary layer upstream of separation. An increase in the amplification rate and the most amplified disturbance frequency is observed in the separatedflow region of the swept configuration and is attributed to boundarylayer conditions at the point of separation that are modified by the spanwise pressure gradient. This results in a slight upstream movement of the location where the shear layer breaks down to smallscale turbulence and modifies the turbulent mixing of the separated shear layer to yield a downstream shift in the timeaveraged reattachment location. The results demonstrate that although crossflow instability does not appear to have a noticeable effect on the development of the transitional separation bubble, the 3D pressure field does indirectly alter the separationbubble development by modifying the flow conditions at separation.